The present invention relates to the manufacture of communication cable such that the cable is sealed against longitudinal migration of water, inside the cable which contains a core and/or conductors that are being electrically insulated by means of a synthetic. Cable of the type to which the invention pertains has usually an assembly of a core being comprised of several conductors, and a sheathing or jacket is arranged around that core while any spaces in between are filled with a sealer. For making such sealed cable one proceeds usually in that the sealing material is forced under pressure in and around the completely stranded cable core under utilization of a pressurized arrangement, for forcing highly viscous sealing material out of a container and through a heatable pressure plate whereby the pressure force is provided through a tube being attached to that plate to provide for the transmission of the requisite pressure force. The plate has a bore being aligned with the axial bore of the tube, and the bore in the plate or the tube includes a pump arranged on the other side with the chamber. The sealing material is thus forced out of the container through the tube.
Generally speaking filling the cavity in and around a cable core, and between the cable core and the jacket in communication cables, by means of a sealing material serves the purpose of avoiding migration of water that may have entered the cable at some point in longitudinal direction. Even though the cable core is insulated electrically, longitudinal migration of water is still to be avoided because the water may find a weak point for example in a connecting sleeve or the like and produce some sort of short circuit. Moreover it was found that a cable into which water has seeped and migrated in the interior for any length thereof deteriorates as to its electrical properties generally.
Devices have been proposed by means of which such a cable is to sealed. As a sealing material, properly termed as a filler, one can use a vaseline-like pasty material being highly viscous at room or normal operating temperature while permitting more or less easy flow and a high rate of fluidity when heated. The temperature dependency of the physical state of the filler requires it to liquify more or less prior to the insertion into the cable or cable core. For a long cable, or one may say for cables to be manufactured in an endless configuration, one needs considerable amounts of energy and equipment just for purposes of liquifying the sealing and filling material.
It is a customary procedure to heat the filler and sealer through an open gas flame, the material being in a suitable container or one uses indirect heating through the container wall. Either procedure is either time consuming and one obtains more or less melting or high degree of liquification in and around the container wall while the center of the container content does not melt at all or only very little. The filler material envisioned here is a rather poor heat conductor. If the entire content of the container is to be melted which is normally to be expected, one must expect for reasons of inadequate convection flow that outer parts of the filling material are overheated. If applied now without any attempt to render the temperature more or less uniform it may well be that certain parts of the filler when applied to the cable are in fact too hot and that may require subsequent cooling.
German Pat. No. 2,405,784 proposes a device wherein as outlined in the introduction herein the basically cold filler is forced under very high pressure out of the container and passes through a flash heater for heating to assume the desired operating temperature. It is not expected to pose any problems that one needs considerable pressure but it is quite possible that when the pressure is too high the container ruptures and that renders temporarily the entire device inoperative. Another problem that was observed is that in the filling material certain air pockets are trapped and remain in the material.
A device still closer to the arrangement alluded to in the introduction is disclosed in German printed application No. 2,614,806. Herein heating elements are provided and completely embedded in the pressure plate which pressure plate has a smooth surface where facing the filler. The heat as developed in the heating elements will therefore not directly affect the filler but the pressure plate has to be heated first. It was found that this indirect heating requires extensive amounts of energy. Moreover, a rapid action response type of any control of the temperature of the heating element is not possible because the large mass of the pressure plate responds very sluggishly to any temperature change. For reduced pump volume or even in case of a stoppage of the entire equipment the filling material again may be overheated just because of the slow response of the temperature control.